Precipitating apparatus



y 1937- y L. D. MILLS El" AL 2,085,820

PRECIPITATING APPARATUS :E 'Il :1 l

.INVENTORS ATTORNEY.

July 6, 19370 D. MILLS ETAL 2,035,820

PRECIPITATiNG APPARATUS Filed Oct. 19, 1934 2 Sheets-Sheet 2 F"- ii 9 3 I T "1 l m i i I i l m l H L II INVENTOR5 [00/3 01*7/7/5 By 760/7755 5. (row:

ATTORNEY.

Patented July 6, 1937 UNITED STATES PATENT OFFICE PRECIPITATING APPARATUS Louis D. Mills and Thomas B. Crowe, Palo Alto, Calif., assignors to The Merrill Company, San Francisco, Calif., a corporation of California Application October 19, 1934, Serial No. 249,068

9 Claims.

' will afford better operating characteristics, particularly with respect to facilitating starting and stopping operations without detrimental consequences. Thus, in general terms, the invention isconcerned with features serving to make such apparatus relatively fool-proof in its operation.

Another object of the invention is to enable automatic pressure control of the solution delivered to the precipitating filter, whereby the precipitating filter may operate under optimum pressure conditions without the possibility of unduly high pressure such as might cause injury to the filter elements or cause ineflicient precipitation.

Further objects of the invention will appear from the following description in which the preferred embodiments of the invention have been set forth in detail in conjunction with the accompanying drawings.

Referring to the drawings, Figs. 1 and'2 are diagrammatic views representing apparatus incorporating the present invention, each view being a different modification.

In the apparatus illustrated in Fig. 1, l0 represents a filter for the clarification of cyanide solution. This filter preferably is one of the vacuum type, having a plurality of removable filter elements ll. Cyanide solution is shown being introduced into the filter by line 12, under the control of a float valve 13. Line 14 represents a suction line for the clarified solution, leading from the filter ill to the deaerating receiver l6. In conformance with established practice, the interior of the deaerating receiver is provided with grids or like means over which the solution is filmed, and the interior of the receiver is normally maintained under a relatively high vacuum by means of vacuum pump IT. The level of accumulated solution in the lower'portion of the receiver 16 is also controlled, as by means of a fioat serving to operate lever l8, which in turn is mechanically connected to the valve IS in line 14.

As pumping equipment for withdrawing the deaerated solution from receiver 16, there is shown a centrifugal pump 2| driven by a motor 22 or like convenient source of power. Immersion of the exterior'of the pump in a liquid bath is desirable, in order to prevent ingress of air,

and has been indicated by a surrounding liquid' container 23. Connection between the inflow side of pump 2| and the lower portion of receiver I8 is indicated by the pipe line 24 which can be throttled by valve 25. An extended handle can be'provided for this valve, for manipulating the same. The outflow or discharge side'of pump 2| is shown connected to a precipitating filter 26, by a flow line designated generally at 21.

While the apparatus can be arranged according to the requirements or conditions of different mills, a convenient positioning has been illustrated with respect to the floor levels I and 2.

As illustrated in Fig. 1, the precipitating filter 26 is of the bag type, having a plurality of separately removable filter bags 28 formed of suitable cloth and adapted to receive solution under pressure. Line 29 represents removal of the barren solution after the precipitated metal has been removed.

The precipitant, which is usually zinc dust, is preferably introduced at some point in the flow line 21-that is, between the pump 2| and the filter 26. As representative of suitable equipment for accomplishing this purpose, there is shown a mixing cone 3| to which measured quantities of zinc dust are introduced by feeder 32. In cone 3| the zinc dust is mixed with a suitable liquid, such as barren solution, which is shown-being introduced by line 33. Simultaneously with the feeding of zinc dust-into the cone, it is irequently desirable to introduce measured quantities of an additional reagent, such as a lead salt solution,

as indicated by feed means 34.

The mixturefrom cone 3| is removed by a pump 36, and is delivered through pipedine 31 to the fiow line 21. In order to normally maintain a proper level within cone 3l,'a return discharge pipe 38 is shown connected to pipe 21, and is provided with a float-controlled valve 29.

ing receiver and thus cause some precipitation of metal within the same. Where the precipitating filter is of the pressure bag type, application of suction has serious consequences, in that 6 it causes a collapsing of the bags, with disruption of any accumulated layer of precipitant and precipitated metal. It has been found that mechanical disturbance or disruption of the layer of precipitate deposited upon the bags, as by col- 10 lapsing of the bags, causes precipitation and removal of the precipitated metal to be relatively less efficient upon reestablishing a normal operation. In order to avoid any possible back-flow of solution to'the deaerating receiver l6, during periods of shut-down or when pump 2| may fail to operate, a check valve 4| is P sitioned in the fiow line 21 at a convenient pointbetween the pump 4| and the point at which the precipitant is introduced through pipe 21. It has been found that such a check valve will function satisfactorily to avoid back-flow, providing it affords a tight seal at all times. However, in actual practice, slight leakage is frequently encountered, or at times the check valve may stick in full open 5 position. To take care of such contingencies, means is provided for venting that portion of the fiow line beyond check valve 4|, so that in the event check 4| fails to operate properly when pump 2| is stopped, air fromthe atmosphere will 30 be sucked through this vent and delivered to receiver l6 to rapidly break the vacuum, thus avoiding application of suction to filter 26.

The venting means described above can be formed by a pipe 42, one end of which is connected to fiow line 21 between check 4| and the point of introduction of precipitant through pipe 31. The other end of pipe 42 is shown extended to a considerable elevation, where it is provided with a down-turned open end 43.

With certain types of precipitating filters, including one of the bag type, it is desirable that the pressure of the solution being delivered to the same be regulated between certain limits. For example, in many installations using pressure bag-type precipitating filters, a pressure main-. tained at about 4 lbs. will give most eificient precipitation and removal of the precipitated metal. If solution is delivered at a pressure considerably in excess of the desired optimum value, the efficiency of precipitation and removal of precipitated metals upon the filter membranes is impaired, or the bags may even burst.

In order to effect an automatic pressure control, and to obviate the possibilities of solution pressure becoming excessive upon the precipitating filter, the vent pipe 42 is also utilized to form an overfiow by-pass back to the clarifying filter l0. Thus, at a suitable elevation above the filter 2 6, and above the fiow line 21, an overflow pipe connects with pipe 42 and leads back to the clarifying filter Ill. During normal operation, a trickle of solution is maintained through pipe 44 back to the clarifying filter. Therefore the maximum pressure upon the precipitating filter is determined by the height of solution sufficient to establish such a slight amount of overflow. Should the fiow resistance to the precipitating filter increase, as is the case shortly before the bags should be removed for washing off the precipitate, a greater amount of solution will flow back to the clarifying filter through pipe 24, without, however, occasioning any material increase in pressure.

Operation of the apparatus described above is as follows: Assuming that the apparatus is being started after a period of shut-down, the vacuum pump I! is started, while the solution fiow line from receiver l6 to the pump 2| is closed and while the solution line from the vacuum receiver l6 to the clarifying filter i0 is open. This causes the solution to be sucked over into the deaerating receiver l6. Such initial solution may not be properly clarified, due to lack of proper conditioning of the filter elements H, and may beimproperly deaerated because of the initial low value of vacuum in receiver l6. When a sufficient quantity of solution has accumulated in receiver l6, the vacuum is reduced somewhat by temporarily venting the line to pump l1, pump 2| is started in operation, andvalve 25 is opened. Because of the lower-than-normal vacuum in receiver IS, a relatively high rate of. delivery from pump 2| will result, which, if delivered directly to the filter 26, might cause serious injury to or bursting of the filter elements of a bag-type filter. However, with the arrangement described, undue pressure upon filter 26 is avoided, because the solution may fiow back through lines 42 and 44, to be redelivered to the clarifying filter II). In fact, during this initial pumping of the solution, the solution line 21 to the filter 26 may be closed, so that all of the solution is returned to the clarifying filter, thus affording an opportunity to condition the clarifying filter with kieselguhr or like filtering aid, and to secure substantially complete deaeration. Assuming that the fiow line to filter 26 has been closed, this line is opened when the operator is assured that the solution is properly clarified and deaerated, and a proper rate of admission of zincfrom cone 3| is established through line 31. The solution fiow from the re-' ceiver l6 through pipe 24 is throttled, until there is only a slight overflow back to the clarifying filter through pipe 44. During a precepitating cycle there is generally an increase in the fiow resistance through precipitating filter 26, particularly during the end of the cycle, dueto accumulation of the precipitated metal upon the filter element 28. Such an increase in fiow resistance can cause no resulting increase-in pressure, because the pressure is limited by the column or head of liquid required to overflow through pipe 44. The operator is immediately apprised of an increase in pressure by virtue of a greater quantity of liquid fiowing back to the clarifying filter through pipe, and can then take steps to properly throttle the fiow line 24 or to replace the precipitating filter elements.

During 'periods of power failure, pumps 2| and 36 cease operating, and because of the vacuum in receiver l6 there is immediately a tendency for solution to be sucked back through pump 2|. As previously pointed out, check valve 4| tends to prevent such flow-back of solution, but in the event this check should fail to operate, or should leak, air is sucked through pipe 42 to break the vacuum, thus avoiding the placing of a suction upon the precipitating filter 26, and likewise avoiding sucking back of solution, together with zinc dust, into the receiver l6.

It is evident that the features described above make for fool-proof starting and stoppiing of the' apparatus, as well as facilitating normal operation. As previously pointed out, the vent established by pipe 42 not only facilitates the breaking of the vacuum in receiver I6, in place of sucking back large quantities of solution and collapsing the bags, but also serves as an automatic pres sure control, which is of particular value with filters of the bag type as disclosed.

With the modification of Fig. 2, in place of securing pressure control by elevated vent and overflow pipes, the solution is delivered to an elevated tank which in turn supplies the filter at a proper gravity head. Thus, in this instance pump 2| delivers solution through the flow line 21a to the tank 41. Sub-level delivery of the solution is employed to insure a quiescent surface and thus prevent re-absorption of oxygen into the deaerated solution. As an added precaution, to prevent contact with the atmosphere, the tank can be covered or sealed. The discharge of solution is shown being controlled by a float valve 48,-whereby a substantially constant liquid level is maintained. Pipe 49 forms a gravity fiow connection between the lower portion of tank 41 and the lower portion of a cone 5|. Zinc dust and an additional reagent are shown being supplied to cone 5| by {the feed means 32 and 34. Line 52 serves to connect the lower end of cone 5| with the precipitating filter 26.

In addition to the pipe 49 connecting tank 41 with the cone 5 a bleed-off line 53 connects with line 21a and is arranged to discharge into the cone. This line is restricted with respect to. fiow through the same, as by providing it with a throttle valve 54. Flow from the cone 5| to line 52 is controlled by a movable valve member 56, which co-operates with a valve seat formed on the inner end of the outflow pipe 52, and which is connected to a float 51. Both tank 41 and cone 5| are disposed at an elevation withrespect to the precipitating filter 26, as by mounting them upon a platform or fioor level 3. The discharge end of flow line 21a may be at a substantial distance below the level of solution in tank 41, to afford a quiescent solution surface, while the discharge from pipe 53 into 'cone 5| is shown slightly above the solution surface.

A valve-controlled pipe line. 58 is shown leading back to the clarifying filter ID from the fiow line 29a. A tank 59 can be provided for receiving barren solution from the precipitating filter.

In general, the modification of Fig. 2 accomplishes the results previously set forth with respect to Fig. 1, although certain additional features are afforded. In starting the apparatus, pipe 58 can be initially opened to by-pass solution to the clarifying filter, thus affording an opportunity to effect complete deaeration of the initial liquid drawn over and to condition the filter elements with a filter aid. Upon closing pipe 58, solution is supplied to tank 41, under the control of fioat valve 48, whereby a substantially constant solution level is maintained. From tank 48 the solution fiows by gravity to the cone 5|, and from thence to the precipitating filter 26 through line 52. Chemicals from feed means 32 and 34 are continuously supplied to the solution in cone 5|. Valve 54 is throttled to establish a small but continuous fiow of solution through pipe 53 to the cone 5|, where it aids in securing proper admixture of zinc dust and reagent from feed means 34, with the solution. Discharge of solution into cone 5| in a region adjacent the valve member 56 is desirable, in that it tends to avoid clogging by zinc dust and at the same time avoids reabsorption of oxygen from the air.

In normal operation, the solution pressure to the filter 26 is definitely maintained at an optimum value, since liquid fiow to the filter is only by virtue of a substantially constant gravity head. The pumping capacity of pump 2| can slightly exceed the normal fiow rate through pipe 52 to the filter 26, but the discharge from the pump is automatically throttled by float valve 48. Solution to one or more. of the bags of the filter 26 can be shut off to enable the bags to be removed, in which event the pressure upon the remaining bags will not be materially increased, but float valve 48 will further reduce the solution discharge rate from pump 2|. In the event of failure of pump 2|, together with failure of check 4| to properly close, line 21a will be vented through pipe 53 to break the vacuum, without disturbing'the solution head upon the precipitating filter, and without sucking back precipitant from the cone 5|. Therefore, during such periods of shut-down the filter bags are maintained distended, and no air is permitted to enter the filter or the pipe lines leading from cone 5|. Upon re-establishing normal operation, any air in flow line 21a or in pump 2| is first expelled, thus again avoiding delivery of air to the filter bags. The slight amount of oxygen reabsorbed by the solution delivered by line 53 is immediately combined with hydrogen evolved by contact of zinc with the alkaline cyanide solution. In any event, the amount of reabsorbed oxygen at this point is slight, because the-fiow of solution through line 53 is very small in relation to the flow of solution through line 48.

We claim: I

1. In apparatus for the precipitation of cyanide solution, filtermeans for clarifying the solution, an evacuated deaerating receiver connected to the filter means for receiving clarified solution for deaeration, a precipitating filter, pumping means having its suction side connected to the deaerating receiver and having its discharge side connected to deliver solution to the I precipitating filterby a solution fiow line, a return pipe line connected to sald'fiow line and leading back to the clarifying filter, and means for automatically venting said solution fiow line to the atmosphere in the event a quantity of solution small in comparison with the volume of the deaerating receiver is sucked back through the pumping means into said receiver and for limiting the solution pressure applied to the precipitating filter.

2. In apparatus for the precipitation of cyanide solution, filter means for clarifying the solution, an evacuated deaerating receiver,connectedto the filter means for receiving clarified solution for deaeration, a precipitating filter, pumping means having its suction side connected to the deaerating receiver and having its discharge side connected to deliver solution to the precipitating filter by a solution flow line, means for introducing a precipitant into the deaerated solution before delivery of the same to the precipitating filter, a check valve arranged in said solution fiow line between said pumping means and the point of introduction of the precipitant, and means for limiting the solution pressure applied to the precipitating filter and for venting to the atmosphere that portion of the fiow line between said check valve and said point of introduction of precipitant, in the event of failure of said pumping means and failure of said check valve to properly close.

3. In apparatus for the precipitation of cyanide solution, filter means for clarifying the solution, an evacuated deaerating receiver connected to the filter means for receiving the clarified solution for deaeration, a precipitating filter of the bag type, centrifugal pumping means having its suction side connected tothe deaerating receiver and serving to deliver solution to the precipitating filter through a flow line, and means associated with the flow line for limiting the solution pressure applied to the precipitating filter and for venting the same in the event the deaerating receiver sucks solution back through the pumping means.

4. In apparatus for the precipitation of cyanide solution, filter means for clarifying the solution, an evacuated deaerating receiver connected to the filter means for receiving the clarified solution for deaeration, a precipitating filter of the bag type, piunping means having its suction side connected to the deaerating receiver, a tank located above the precipitating filter, a pipe serving to introduce solution from the pumping means to said tank, a fioat valve operated by the liquid level in said tank and serving to control the solution discharge from said pipe, a second tank located at substantially the same level as the first tank and having a solution fiow connection with the same, means for introducing precipitant into the second tank, a gravity flow connection between the second tank and the precipitating filter, and a combination vent and bleed-off pipe connected to the solution pipe from the pump and discharging into the second tank.

5. In apparatus for the precipitation of cyanide solution, filter means for clarifying the solution, an evacuated deaerating receiver connected to the filter means for receiving the clarified solution for deaeration, a precipitating filter of the bag type, pumping means having its suction side connected to the deaerating receiver, a tank located above the precipitating filter, a pipe serving to introduce solution from the pumping means tosaid tank, a float valve operated by the liquid level in said tank and serving to control the solution discharge from said pipe, 9. second tank located at substantially the same level as the first tank and having a solution fiow connection with the same, means for introducing precipitant into the second tank, a gravity fiow connection between the second tank and the precipitating filter, a float-controlled valve operated by the liquid level in the second tank and serving to close the fiow line to the precipitating filter in the event said level falls below a predetermined value,.and a combination vent and bleed-ofi pipe connected to the solution pipe from the pump and discharging into the second tank;

6. In apparatus for the precipitation of cyanide solution, filter means for clarifying the solution, an evacuated deaerating receiver connected to the filter means for receiving the clarified solution for deaeration, a precipitating filter of the pressure bag type, pumping means having its suction side connected to the deaerating receiver and serving to deliver solution to. the precipitating filter through a fiow line, and a pipeline having one end of the same connected to said flow line and extended to an elevation above the same, the other end of said flow line being open and arranged to return solution to the clarifying filter, said pipe line serving to establish a constant pressure head of solution on the precipitating filter.

a '7. In apparatus for the precipitation of cyanide solution, filter means for clarifying the solution, an evacuated deaerating. receiver connected to the filter means for receivingthe clarified solution for deaeration, a precipitating filter of the pressure bag type,- pumping means having its suction side connected to the deaerating receiver and serving to deliver solution to the precipitating filter through a flow line, and a pipe line having one end of the same connected to said flow line and extended to an elevation above the same, the other end of said flow line being open and arranged to return solution to the clarifying filter, said pipe line serving to establish a c0nstant pressure head of solution on the precipitating filter, and a check valve interposed in the flow line between said pump and the point of connection of-the pipe with the flow line.

8. In apparatus for the precipitation of cyanide solution, filter means for clarifying the solution, an evacuated deaerating receiver connected to the filter means for receiving the clarified solution for deaeration, a precipitating filter of the pressure bag type, pumping means having its suction side connected to the deaerating receiver and serving to deliver solution to the precipitating filter through a flow line, and a pipe line having one end of the same connected to said flow line and extended to an elevation above the same, the other end'of said fiow line being open and arranged to return solution to the clarifying filter, said pipe line serving to establish a constant pressure head of solution on the precipitating filter, a check valve interposed in the flow line between said pump and the point of connection of the pipe with the flow line, and means for introducing a precipitant into the flow line at a point intermediate the precipitating filter and the point of connection between said pipe and the fiow line.

9. In apparatus for the precipitation of cyanide solution, filter means for clarifying the solution, an evacuated deaerating receiver connected to the filter means for receiving clarified solution for deaeration, a precipitatingv filter, pumping means having its suction side connected to the deaerating receiver and having its discharge side connected to the precipitating filter by a solution flow line, and means for automatically venting saidsolution fiow line to the atmosphere in the event a quantity of solution small in comparison to the volume of the receiver, is sucked back through the pumping means into the deaerating receiver, said means also serving to limit the pressure of solution supplied to the precipitating filter.

LOUIS D. MILLS. THOMAS B. CROWE. 

